The present invention generally relates to a communication control method for a communication system in which a plurality of communication control apparatuses are interconnected mutually by way of a communication line. More particularly, this invention is concerned with a communication control method for protecting a communication control apparatus from being undesirably left in an occupied or locked state incapable of any further communication when a failure occurs in a counterpart communication control apparatus with which communication has been performed in the locked state.
For affording a better understanding of the present invention, problems in a communication system (an improvement of which this invention concerns) will be described in some detail. FIG. 2 of the accompanying drawings shows in a block diagram a conventional communication system in which a plurality of communication control apparatuses are mutually interconnected by way of a communication line 1. The communication control apparatus 21 is composed of a packet transmission/reception unit 21a and a controller 21b. Similarly, the communication control apparatus 22 includes a packet transmission/reception unit 22a and a controller 22b, and the communication control apparatus 23 is constituted by a packet transmission/reception unit 23a and a controller 23b. When the communication control apparatus 21 performs communication with the communication control apparatus 22, the controller 21b of the communication control apparatus 21 first supplies communication data to be transmitted to the packet transmission/reception unit 21a. Upon reception of the communication data, the packet transmission/reception unit 21a sends out the data in the form of packets for transmission onto the communication line 1. The packet transmission/reception unit 22a of the communication control apparatus 22 then receives the packets from the communication line 1 and transfers the corresponding communication data to the controller 22b which receives the data as the communication data from the communication control apparatus 21. In the communication system shown in FIG. 2 in which communication is carried out in the manner mentioned above, each of the communication control apparatuses performs occupation control or lock control of the counterpart or addressee communication control apparatus in precedence to data communication. More specifically, an addresser communication control apparatus transmits a lock (occupation) request packet to an addressee communication control apparatus to which the addresser apparatus desires to transmit data, in precedence to the transmission of the data. Upon reception of the lock request packet by the addressee communication control apparatus, the latter is set to a locked state in which the addressee communication control apparatus can no more receive communication packets from any other communication control apparatus than the addresser apparatus which had caused the locked state. Upon completion of the required data transmission, the addresser communication control apparatus finally sends an unlock request to the addressee communication control apparatus to thereby release the latter from the occupied or locked state. By way of example, an explanation will be made of a case where the communication control apparatus 21 communicates with the communication control apparatus 22, by making reference to FIG. 5. In that case, the addresser communication control apparatus 21 sends a lock request packet 3 to the addressee communication control apparatus 22 in precedence to data transmission, as is illustrated in FIG. 5. Upon reception of the lock request packet 3, the communication control apparatus 22 is locked to a state incapable of receiving any communication packet sent from other communication control apparatuses than the addresser communication control apparatus 21. In this state, the addresser communication control apparatus 21 sends data to be transmitted to the addressee communication control apparatus 22 in the form of communication messages 51 and 52. In the course of this communication, even if a communication message 11 is sent to the communication control apparatus 22 from the communication control apparatus 23, the former is not in the state capable of receiving the communication message 11 because the communication control apparatus 22 is locked or occupied by the communication control apparatus 21. Only after the communication control apparatus 21 sends an unlock (lock release) request 9 to the communication control apparatus 22 and the communication control apparatus 22 recognizes by this unlock request that it has been released from the locked state is the communication control apparatus 22 brought into the position to receive the communication message 11 from the communication control apparatus 23. The data transmission among the communication control apparatuses is realized through the lock control procedure as described above.
However, in a conventional communication system described above, there may arise such a problem that the communication control apparatus 22 set to a locked state in response to the reception of the lock request packet 3 from the communication control apparatus 21 is permanently left in the locked state, if the addresser communication apparatus 21 becomes incapable of performing any further communication with the addressee communication apparatus 22 because of power-off or some other failure occurring in the addresser communication apparatus 21 during the transmission of messages 51 and 52 therefrom, as illustrated in FIG. 6. In other words, the communication control apparatus 22 is rendered to be permanently in the state incapable of receiving communication packets after the occurrence of a failure in the addresser communication control apparatus 21. Thus, neither a messages 10 nor a message 11 can be received by the address communication control apparatus 22.